1. Technical Field of the Invention
The present invention relates generally to a gas sensor which may be installed in an exhaust system of an internal combustion engine to determine the concentration of O2, an air-fuel ratio, or the concentration of NOx in exhaust emissions, and more particularly to an improved structure of such a type of gas sensor designed to ensure a rapid response to a change in, for example, concentration of a gas to be measured.
2. Background Art
Conventionally, gas sensors are used for burning control of internal combustion engines for automotive vehicles. As a typical example, a gas sensor is installed in an exhaust pipe of an automotive engine to measure the concentration of a specified gas contained in exhaust emission of the engine. A gas sensor of this type consists essentially of a gas sensor element disposed within a hollow cylindrical housing, an air cover installed on a base portion of the housing, and a gas cover assembly installed on a tip portion of the housing which is made up of an outer cover and an inner cover.
FIGS. 16 and 17 show an example of such a gas cover assembly. The gas cover assembly 9 is made up of an outer cover 91 and an inner cover 92. The inner cover 92 defines therein a gas chamber 900 into which a gas to be measured is admitted through gas inlet holes 210 and 220. The gas inlet holes 220 formed in the inner cover 92 are located closer to a base portion of the gas sensor (i.e., an upper side of the drawings) than the gas inlet holes 210 formed in the outer cover 91. The gas to be measured first enters the gas inlet holes 210 of the outer cover 91, creates an upward flow 81, as indicated by an arrow in each drawing, within a clearance 25 between the covers 91 and 92, and then enters the gas chamber 900 through the gas inlet holes 220 of the inner cover 92.
The above structure, however, encounters the drawback in that a portion of the gas not admitted into the gas inlet holes 220 returns on an upper end of the clearance 25, as viewed in the drawings, downwards, forming a return flow toward the upstream end of the clearance 25, which disturbs entrance of the gas into the gas chamber 900 through the gas inlet holes 220. This results in a time lag between measurement of, for example, the concentration of the gas admitted into the gas chamber 900 and a change in concentration of a gas existing outside the cover assembly 9, thus leading to a decrease in response rate of the gas sensor.
It is therefore an object of the invention to provide an improved structure of a gas sensor designed to ensure a rapid response to a change in, for example, concentration of a gas to be measured.
According to one aspect of the invention, there is provided a gas sensor which may be employed in measuring the concentration of a specified gas contained in exhaust emission of an internal combustion engine of an automotive vehicle. The gas sensor has a length with a first and a second end opposed to each other and comprises: (a) a hollow cylindrical housing having a first end and a second end opposed to the first and second ends of the gas sensor, respectively; (b) a sensing element retained within the housing which has a length extending in a longitudinal direction of the gas sensor, including a sensing portion working to measure a concentration of a specified gas; and (c) a cover assembly installed on the second end of the housing to define a gas chamber in which the sensing portion of the sensing element is disposed and into which the specified gas is admitted. The cover assembly has a length extending in the longitudinal direction of the gas sensor and includes an outer and an inner cover. The outer cover has a first gas inlet hole formed in a side wall thereof. The inner cover has formed in a side wall thereof a second gas inlet hole which is located closer to the first end of the gas sensor than the first gas inlet hole of the outer cover. The inner cover is disposed within the outer cover with a given clearance between the outer and inner covers. At least one of the outer and inner covers is geometrically designed to define a gas path within the clearance which establishes a flow of the specified gas from the first gas inlet hole to the gas chamber through the second gas inlet hole for minimizing the interference of a return gas flow produced within the clearance, oriented toward the first gas inlet hole with the flow of the specified gas into the gas chamber along the gas path. This facilitates ease of the entrance of the flow of the specified gas into the gas chamber, thereby ensuring a rapid response of an output of the gas sensor to a change in concentration of the specified gas.
In the preferred mode of the invention, one of the outer and inner covers of the cover assembly has formed on the side wall thereof a shoulder which separates the clearance between the outer and inner covers into a wider and a narrower portion.
The wider portion is preferably 1.1 times wider than the narrower portion.
The wider portion is located around the second gas inlet hole of the inner cover.
The inner cover may have a gas flow-opposed wall oriented upstream of the gas path. The second gas inlet hole may be formed in the gas flow-opposed wall.
The gas flow-opposed wall may be defined by a shoulder which is formed on the side wall of the inner cover and extends outward to the first end of the gas sensor.
A groove may also be formed in the side wall of the outer cover serving to direct the flow of the specified gas to the second gas inlet hole of the inner cover.
According to the second aspect of the invention, there is provided a gas sensor having a length with a first and a second end opposed to each other which comprises: (a) a hollow cylindrical housing having a first end and a second end opposed to the first and second ends of the gas sensor, respectively; (b) a sensing element retained within the housing which has a length extending in a longitudinal direction of the gas sensor, including a sensing portion working to measure a concentration of a specified gas; and (c) a cover assembly installed on the second end of the housing to define a gas chamber in which the sensing portion of the sensing element is disposed and into which the specified gas is admitted. The cover assembly has a length extending in the longitudinal direction of the gas sensor and includes an outer and an inner cover. The outer cover has a first gas inlet hole formed in a side wall thereof. The inner cover has a tapered surface formed on a side wall thereof which tapers off to a side of the second end of the gas sensor. The inner cover has formed in the tapered surface thereof a second gas inlet hole which is located closer to the first end of the gas sensor than the first gas inlet hole of the outer cover. The inner cover is disposed within the outer cover with a given clearance between the outer and inner covers which defines a gas path establishing a flow of the specified gas from the first gas inlet hole to the gas chamber through the second gas inlet hole. The tapered surface geometrically serves to minimize the interference of a return gas flow produced within the clearance, oriented toward the first gas inlet hole with the flow of the specified gas into the gas chamber along the gas path.
In the preferred mode of the invention, the outer cover has formed on the side wall thereof a surface which tapers off to a side of the first end of the gas sensor to define a wider portion of the clearance between the tapered surface of the inner cover and the tapered surface of the outer cover.
The inner cover has a straight surface continuing from the tapered surface, extending straight in the longitudinal direction of the gas sensor.
A groove may also be formed in the side wall of the outer cover serving to direct the flow of the specified gas to the second gas inlet hole of the inner cover.
The outer cover may have a shoulder formed closer to an upstream side of the flow of the specified gas than the tapered surface of the inner cover to define an upstream portion of the gas path closer to the first gas inlet hole of the outer cover and a downstream portion of the gas path closer to the second gas inlet hole of the inner cover. The downstream portion is greater in area than the upstream portion in a direction traversing the longitudinal direction of the gas sensor.
A groove may also be formed in a portion of the side wall of the outer cover closer to the first gas inlet hole than the shoulder of the outer cover. The groove serves to direct the flow of the specified gas to the second gas inlet hole of the inner cover.
The groove may have a length extending in alignment with the second gas inlet hole of the inner cover in the longitudinal direction of the gas sensor.